A fundamental question in immunology is how immune cells recognize and mount a robust response to invading microorganisms. Genetically tractable pathogens capable of triggering distinct responses in immune cells are extremely useful in determining the microbial ligands or activity responsible for such responses and the corresponding host immune surveillance mechanisms. Legionella pneumophila is a ubiquitous bacterium, which in the environment is primarily associated with amoebae as an intracellular parasite. This bacterium also can replicate within mammalian alveolar macrophages and causes Legionnaires' disease, a severe inflammatory pneumonia. The amoebae host is believed to provide the primary or exclusive evolutionary pressure for its acquisition and maintenance of bacterial traits necessary for intracellular replication. Thus, L. pneumophila appears not to have evolved sophisticated immune-evasive mechanisms and can serve as an excellent model for identifying bacterial ligands or activity sensed by the host immune system. We found that an environmental isolate of L. pneumophila (strain LPE509) was unable to replicate in bone marrow-derived macrophages from A/J mice, which are permissive for intracellular replication by most, if not all examined clinic L. pneumophila isolates. Strain LPE509 grows robustly in the amoebae host Dictyostelium discoideum and the differentiated human macrophages U937 cells in a process that is dependent upon the Dot/Icm type IV secretin system, which is required for the virulence of all examined L. pneumophila strains. Further, we found that LPE509 induced extensive cell death in A/J macrophages in a process that required the Dot/Icm transporter but not flagellin or the NAIP5-NLRC4 inflammasome. Thus, we hypothesized that strain LPE509 codes for one or more factors that can be recognized by the host immune surveillance system, leading to host cell death. In this proposal, we will test this hypothesis by pursuing two specific Aims: First, we will identify the bacterial factor(s) responsible for the induction of cell death i A/J macrophages. Second, we will determine the host pathway involved in the recognition of the bacterial signal(s).